1. Field of the Invention
The invention relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, a separator provided between the positive electrode and the negative electrode, and a non-aqueous electrolyte held at least in the separator.
2. Description of Related Art
Japanese Patent Application Publication No. 2002-319386 (JP 2002-319386 A) describes providing a non-aqueous electrolyte secondary battery with a separator obtained by stacking a plurality of layers each made of a porous material having microscopic pores. In the separator described in JP 2002-319386 A, at least two of the layers each made of a porous material are different in porosity from each other. Among the layers that are different in porosity from each other, the layer having the highest porosity is greater in mean pore size of the microscopic pores than the layer having the lowest porosity. According to JP 2002-319386 A, using such a separator makes it possible to achieve compatibility between good low-temperature characteristics and high overcharge safety in the non-aqueous electrolyte secondary battery.
In a non-aqueous electrolyte secondary battery, a separator is provided between a positive electrode and a negative electrode, and a non-aqueous electrolyte is present inside a plurality of pores of the separator. This configuration ensures the migration of ions (ions contained in the non-aqueous electrolyte (e.g. lithium ions)) between the positive electrode and the negative electrode or the flow of electric current between the positive electrode and the negative electrode.
When the temperature of a non-aqueous secondary battery reaches or exceeds the melting point of a material of a separator of the non-aqueous secondary battery, the material of the separator melts. As a result, pores of the separator are blocked, so that the migration of ions is interrupted or the flow of electric current is interrupted (the separator is shut down). A shutdown of the separator increases the resistance between a positive electrode and a negative electrode, thereby preventing a large current from flowing between the positive electrode and the negative electrode. Thus, the operation of the non-aqueous electrolyte secondary battery is safely stopped.